In the field of materials handling, it is frequently desired to transport articles from one location to another. One method of doing so that is particularly adapted to handling articles which are flexible, limber or otherwise difficult or impossible to grasp is to pick them up by means of vacuum. Typically, such devices include a vacuum box having an apertured bottom about which an endless, foraminous belt moves by means of drive rolls and idler rolls. By this means, articles that have been moved into position beneath the vacuum box will become suspended from the underside of the conveyor belt, by which they may be moved from one location to another. Thus, for example, it may be desired to transport tortillas after they have been cooked and cooled, from the top of a moving conveyor belt to stacking apparatus in which piles of tortillas are accumulated for packaging and/or further processing. Particular problems with tortillas are that the distribution of the material comprising each tortilla typically is markedly uneven as between different regions of the same tortilla, and each tortilla tends to differ significantly in its weight as between tortillas. These characteristics tend to exaggerate the undesirable effects which occur when vacuum transfer mechanisms are used of the types heretofor known.
Such prior art apparatus typically has included a variety of means for disassociating the articles being carried from the vacuum-implemented conveyor belts from which they are suspended and are being moved. Scraper blades have proven inadequate because they cause the leading edge of the tortilla to start to fall first, resulting frequently in the tortilla being folded under, or wrinkled when it comes to rest on the surface to which it falls. Prior art efforts to interrupt the vacuum as a means to achieve release of the product have also proved unsatisfactory, particularly as production speeds have increased. One proposal was to simply eliminate periodically the vacuum in the vacuum box. This is a costly and time consuming approach because of the necessity for repeatedly re-establishing the vacuum within the box. Further, the effective distance of conveyance is severely limited by reasonable production volumes since, without more, other articles suspended on the belt of such apparatus will drop simultaneously with the one desired to be released. Neither has it proved feasible, in an attempt to circumvent these difficulties, to achieve release by having the articles move past the vacuum box section into a region of no vacuum or positive pressure. That is because each article as it is moved along is released progressively from front toward back, rather than simultaneously throughout, so that, just as with a scraper blade, the front edge tends to fold under, the tortilla tends to land wrinkled, etc. Further, unless positive pressure is introduced to overcome the vacuum in each aperture of the belt, the presence of the tortilla and the sealing effect on it of the vacuum previously introduced into each aperture tends to cause the vacuum to linger for a while at least. The rate of decay of the vacuum will then vary according to a number of factors, such as how effective the seal is between the bottom of the belt and each tortilla. This coupled with the significant irregularity of weight as between tortillas means that the lapsed time period following interruption of the vacuum before release will occur may vary widely. With the belt moving at the comparatively high and sustained linear speeds that are highly desired in the interests of high production volumes and efficiencies, the tortillas drop at widely distributed locations along the path of travel, making such desired results as orderly, straight stacking impossible.
It has also been proposed to position a flat plate having apertures corresponding to those of the bottom of the vacuum box immediately above the bottom in contacting sliding relation thereto. When vacuum is desired through the belt, the plate is positioned so that its apertures are aligned with those of the box bottom. Sliding the plate laterally may then cause the rapid and simultaneous shutting off of a multiplicity of the box bottom holes. Even this, however, has not proved satisfactory because of the problem of residual vacuum in the carrier belt apertures, with consequent unsatisfactory stacking as described above, and because of the great leakage between the plate and the box bottom which higher vacuums will induce.
Accordingly, it is an object of this invention to provide means for releasing satisfactorily objects being transported by a vacuum box conveyor.
Another object of this invention is to provide such means adapted particularly for the release of flexible articles being so conveyed.
Still another object of this invention is to provide means by which objects transported by a vacuum conveyor may be automatically and accurately stacked.